1. Field of the Invention
2. Discussion of the Related Art
Applying a powder coating using a gas-carrier flow to entrain powder particles and spraying the powder to a surface is known in the art. The powder is mixed with gas-carrier flow and the formed gas-powder flow is accelerated in a nozzle and directed to an article. Two prior art patents discussed below describe the current state of the art.
In U.S. Pat. No. 4,815,414, a method is disclosed which utilizes an apparatus consisting of a compressed air supply connected to a powder feeder equipped with a powder bunker and a dosing device and a mixing chamber. The inlet of the mixing chamber is connected to the dosing feeder, and its outlet is connected to the group of accelerating nozzles via a distributing collector.
The method of the '414 patent is limited in that it is capable of only producing powder coatings having small thickness, since the gas-powder jet directed to an article has the temperature nearly equal to the temperature of the environment. In this case the efficiency of this method is limited to the group of thermoplastic polymers, which are applied to an article at cold state. In order to achieve a better cohesion of powders with the surface, it is required to heat the article to the temperature of the melting point of the applied material. It makes this process considerably difficult to implement and limits its application. In addition, the utilization of a distributing collector in the apparatus inevitably causes unequal distribution of the gas-powder mixture between the nozzles. Furthermore redistribution of the main gas-carrier flow between the nozzles severely reduces the energy of the flow in each nozzle, which involves reduction of the discharge of the carried gas-powder mixture, and accordingly reduces the efficiency of the process.
In WTO Patent 91/19016, a method of applying powder coatings is disclosed comprising a heated gas-carrier flow, a powder selected from a group of metals, their blends and dielectrics, with the particle size 1-50 μm, mixing with a gas-carrier flow, where the generated gas-powder flow is accelerated inside a nozzle and then a supersonic jet of the preset profile is formed and directed to a surface. In this case the supersonic jet of the preset profile is generated by the way of linear gas expansion. The apparatus for implementing the above method contains a compressed air supply connected by a gas pipe with a heating unit; a mixing chamber is connected to the powder feeder equipped with a powder bunker and a dosing device. The inlet of the mixing chamber is connected to the intermediate nozzle, and its outlet to the accelerating nozzle.
The disadvantage of this method is that it is efficient only for the particles of the small size, in particular 1-50 μm. Generation of the preset profile of the gas-powder jet by the way of the linear expansion of the gas is reasonable only for the small size particles, since the unbalance of the profiles of the gas jet and powder parameters dramatically increases as the particles size increasing. Moreover, a problem with the prior art device is that small powder particles are quickly oxidized in active gas surrounding. Thus, when the gas jet temperature increases, the produced coatings have high-porous, heterogeneous and thermally stressed structure, which reduces the quality of the applied coating. Therefore, the generation of the preset profile of the gas-powder jet becomes no more efficient than the prior art. Further, generation of the profile of gas-powder jet by the way of linear expansion requires the accelerating nozzle of considerable length, within the limits of which particles of gas-powder jet are accelerated until their threshold value, at which particles deposit and cohere to an article. The increase of the size of the accelerating nozzle thereby causes the increase of the size of the apparatus itself, and therefore limits its application (e.g. in case of applying coatings to inner surface of articles).
Thus, there is a need in the art to provide improved method and apparatus of applying a coating of powder material with a wider range of particle sizes, improving the quality of the applied coating, and reducing the size of the spraying apparatus to allow for increased applicability.